Cos^4a=(3\8)+(1\2)*cos2a+(1\8)*cos4a

Cos(4a) = 2cos^2(2a) - 1 = 2(2cos^2(a) - 1)^2 - 1 = 8cos^4(a) - 8cos^2(a) + 2 - 1 = 8cos^4(a) - 8cos^2(a) + 1 можно выразить через sin(a